Generating clean power from renewable sources to meet all or most of world's energy needs is a goal that will not occur without a transformational change in energy harvesting methods. Among the available options, solar energy stands out as a ubiquitous renewable source. The solar (light) energy is converted into useful chemical energy (food) by photosynthesis, a process that evolved over 2.5 billion years. Mimicking or engineering this biological phenomenon could provide a renewable alternative energy technology that is nature-inspired, clean, environment friendly and self-sustainable.
Plant photosynthesis provides an unmatched quantum efficiency of nearly 100%. In recent years, significant interest has evolved in mimicking and/or harnessing the photosynthetic process for energy conversion and hydrogen generation applications. Multiple approaches to artificial photosynthesis exist, including light energy harvesting using natural pigments from plants and microorganisms and using whole cell microorganisms. Scientists have explored photosynthetic organelles such as thylakoids, chlorophyll molecules, photosystems, and oxygen evolving complexes for photo-electrochemical activity. However, isolated organelles tend to lack the stability of whole cells or organisms.
Microorganisms capable of transferring electrons to extracellular electron acceptors as a part of their metabolism are called exoelectrogens. The extracellular electron transfer (EET) capability of microorganisms has been exploited in microbial fuel cells (MFC) to generate electricity using organics as fuel. When deprived of their cellular terminal electron acceptor, these microorganisms prefer to dump their electrons to the electrode, thereby using it as an alternate electron sink. In MFC, the commonly used exoelectrogens such as Geobacter sp. and Shewanella sp. consume organic carbon sources such as acetate or lactate as their reductant and transfer the electrons from their respiratory electron transport chain to the anode while simultaneously releasing CO2. On the contrary, photosynthesis based electricity generation using cyanobacteria requires only light and water without the need of any external organic carbon source and therefore offers huge potential for development of a clean, renewable and environmentally friendly alternate energy source. Cyanobacteria have inherent ability to perform EET and are shown to generate electricity in photo-bioelectrochemical fuel cells. However, the current densities of photo bio-electrochemical cells (PBECs) and/or photosynthetic microbial fuel cells (PMFCs) based on photosynthetic microorganisms such as cyanobacteria generate current densities lower than that of standard solar cells, and the power density achieved using cyanobacteria is roughly two orders of magnitude lower compared to that produced by the exoelectrogens in MFC. Thus, cyanobacteria based PBECs and PMFCs are currently not competitive against biofuel cell and photovoltaic technologies or standard solar cells.